The present invention concerns a transmitting and receiving circuit for ultrasonic flowmeters of the kind described in the preamble of claim 1.
In such circuits, it is known that measuring the very small time differences between the transmission of ultrasound upstream and downstream requires a very high degree of symmetry in the electronic circuits used for transmitting and receiving ultrasonic signals via the transducers, so that the group running time for signals in the electronic circuit itself is the same for both upstream and downstream signal transmission. It is also known that the impedances loading each transducer must be constant from the transmitting situation to the receiving situation to meet the condition of reciprocity of the sensor. This causes the upstream and downstream transmission times of the sensor to be equal when the medium stands still.
This problem is treated in WO 94/17371, which describes the use of two identical transmitting and receiving circuits, one for each transducer, which comprise an amplifier with a first input terminal for connection to an ultrasonic flow transducer, a second input terminal for connection to a signal source, an output terminal for connection to a detection circuit and a feedback connection between the output terminal and the first input terminal. Each of the circuits is able to work as a transmitting circuit or a receiving circuit, and their mode of functioning is switched by means of a switching arrangement or by switching both circuits between the transmitting and receiving function to obtain the transmission of ultrasonic signals in both directions. However, variations caused by component tolerances, different temperature coefficients and the like between two such circuits, will still enable both fixed and temperature dependent group running time differences for signals in the electronic circuits.
On the basis of this state of the art, the purpose of the present invention is to provide a transmitting and receiving circuit for ultrasonic flowmeters as mentioned in the introduction, with which it is possible to solve the problems with variations between two such circuits caused by component tolerances, different temperature coefficients and the like, and at the same time to obtain a simplification of the circuit used, and this purpose is met by means of a circuit of the kind mentioned, which, according to the present invention, also comprises the measures stated in the characterising part of claim 1. This arrangement provides a two-way transmission of ultrasonic signals with merely one single transmitting and receiving circuit of the kind mentioned, as the switching means in turn connect one of the ultrasonic transducers functionally with the first input terminal. With the arrangement described, component tolerances, different temperature coefficients and the like, will be equal for the transmission of ultrasound in the two opposite directions, so that the group running time for signals in the electronic circuit itself is equal for the transmission of ultrasound in the two opposite directions. This applies for both fixed and variable group running time differences, where the fixed differences are primarily caused by component tolerances and the variable differences are caused by temperature dependent parameters. When the fixed group running time difference is eliminated to zero, it is obtained that the zero point calibration/adjustment can be avoided, which removes error possibilities and saves time. For reasons of, for example, authority approvals, the temperature dependent group running time difference must be kept within fixed limits. When transmitting ultrasonic signals in a first direction, the circuit is connected with the first ultrasonic transducer by way of the switching means, while a signal is transmitted to the transducer and converted to an ultrasonic signal. Then the circuit is switched to be connected with the second ultrasonic transducer for receipt of the signal, which will be generated, when the second transducer receives the signal transmitted from the first transducer. When transmitting the ultrasonic signal in the opposite direction, the circuit is first connected with the second transducer for transmission and subsequently with the first transducer for reception.
Preferred embodiments according to the invention are described in the subclaims, suggesting different placings of the ultrasonic transducers and the switching means, preferred frequency ranges for the ultrasonic transducers and the opportunity of using more than two ultrasonic transducers.